The steerability of an icebreaker or a supply ship working in ice-covered water is an extremely important characteristic. Traditionally, such a vessel is equipped with one or more propellers and a rudder located behind the propeller. An extremely efficient alternative propulsion and steering arrangement is a turnable steering thruster device. The patent FI 94508 presents one solution, in which the vessel moves under difficult ice conditions stern forward utilizing steering thrusters.
As the main propellers of a ship can be used traditional propeller shaft solutions, in which the driving mechanism inside the hull of the vessel rotates the propeller via a propeller shaft that is led through the hull and is often quite long. To increase the efficiency of steering, under these circumstances, it is common to equip the vessel with a thruster installed in a transverse tunnel, by which is provided a transverse water flow and steering reaction force. These kinds of propellers are generally installed in the bow of a vessel, but also in the area of the stern, in a middle skeg or side skegs independent of the ordinary propeller shaft lines.
In steering thruster solutions, neither tunnel thrusters nor rudders are usually needed. The propeller drives are arranged either inside a turnable pod, or the operation is arranged via shafts and angle gears by a motor located in the hull of the ship.
It is obvious that the dimensions and structure of the components, which will be in contact with ice, must be selected taking into consideration the stress created by the ice.
Although a steering thruster device is quite efficient solution for propulsion device and for steering of a ship, cases do occur, in which significant problems are related to its use. Such cases are the loads created by particularly thick and/or multi-year ice. In these cases, an arrangement is often selected, which has one or more traditional propellers and propeller shafts. To assure steerability, also at a slow travelling speed and when the ship is stationary, it is, in this case, necessary to use transversely installed tunnel thrusters.
FIGS. 1a, 1b and 1c show an arrangement, which has two shaft lines 2 and 3 and one middle skeg 6, in which is located two tunnel thrusters 7a and 7b. In this arrangement, the operating ability of the vessel in ice is weakened due to the fact that the middle skeg hinders the free movement of broken ice chunks. Support from below for the rudder and a possible propeller nozzle is also difficult to implement, which is likely to weaken the ability of the structure to withstand ice chucks when travelling in icy waters.